1. Field of the Invention
The present invention relates to ceramic composites with high-heat resistant properties for use where high-heat resistant properties are required, and in particular to the process of preparing ceramic composites with high-heat resistant properties, resistance to oxidation and excellent strength under increased temperatures. More specifically, the present invention relates to a process of preparing castable self-lubricating ceramic composites of low porosity cable of maintaining hot-hardness temperatures in excess of 750xc2x0 C.
The composites derived from this process comprise a combination of ceramic powders, cemetitious binders, and solid lubricants e.g. MoSi2 and CdO which are particularly useful in manufacturing high-performance turbine engines including various engine parts such as bearings, gears, rotors and the like.
2. Background of the Invention
Ceramic composites are useful in diverse applications such as engine components, cutting tools and various wear resistant parts. Ceramic composites are known to have improved fracture toughness and improved wear properties. Conventional ceramics are generally monolithic materials which have low fracture toughness. This makes these ceramics brittle and they are liable to crack under stressed conditions, and therefore are not very useful for diverse industrial or military applications.
In general, however, ceramics have excellent heat resistance, oxidation resistance and corrosion resistance characteristics and are desirable as structural materials. Among the ceramics, silicon carbide (SiC) and silicon nitride (Si3N4) particularly are excellent as heat resistance and oxidation resistance materials and are very desirable as structural materials capable of being used at high temperatures. However, SiC and Si3N4 are materials that are difficult to provide in a dense sintered form without adding assistants. For example, Al2O3 is added as a sintering aid to SiC powder or to Si3N4 powder. Thus, there has been extensive research to produce hot-hardness ceramic composites of higher fracture toughness using silicon nitride or silicon carbide reinforced by other materials such as alumina, tungsten carbide, titanium carbide and the like.
There is a program which has a goal to construct an advanced engine. This engine should have a 20:1 thrust-to-weight ratio representing a 100% increase and a 50% decrease in fuel consumption over the state-of-the-art propulsion systems. The payoffs on attaining this goal are sustained mach 3+ capability and a 100% increase in range, loiter and payload. One of the limiting materials technology that currently exists in achieving these capabilities centers on providing lubrication to bearing surfaces. Tribo-materials, i.e. bearings, gears, rotors, and the like are required to operate at temperatures as high as 750xc2x0 C. for the life of the system. There currently exists a major technology gap in materials and lubricants capable of functioning as friction couples at these extremely high temperatures, which if not bridged, will severely limit the successful achievement of this goal.
As propulsion system temperatures increase, a greater demand is placed on lubricant-bearing systems in moving mechanical assemblies such as bearings, gears and rotors. At elevated temperatures, liquid and grease lubricants evaporate, thermally decompose and oxidize. Currently available solid film lubricants are limited also to 650xc2x0 C. and are short-lived, requiring constant replenishment. Currently used M-50 metallic bearings soften at 320xc2x0 C. and thus cannot support load. Therefore, the need for the development of advanced materials technology to eliminate these shortcomings is readily apparent.
In accordance with the present invention, a primary purpose is to advance the state-of-the-art of lubricating ceramic materials for moving components under load such as bearings, and to provide a chemical system which contributes to enhancement of lubrication at ambient and elevated temperatures.
More specifically, this invention relates to a process and to the products derived therefrom which comprises a self-lubricating ceramic composite of low porosity comprising a mixture of:
(a) about 50 to 80 parts by weight of at least one ceramic powder having a particular size of less than 5.0 microns and preferably selected from the group consisting of silicon nitride, silicon carbide, zirconia (zirconium oxide), alumina (aluminum oxide), zirconium nitride, tungsten carbide, and titanium carbide,
(b) about 0.1 to 10 parts by weight of a cementitious binder such as hydraulic cement (calcium aluminate),
(c) about 0.1 to 10 parts by weight of at least one metal silicide such as MoSi2,
(d) about 0.5 to 10 parts by weight of at least one metal oxide such as CdO, and from
(e) about 0 to 30 parts by weight of water. Either a dry mixture or a water slurry of the ceramic powders are subsequently subjected to pressures of about 6.0 to 7.0 MPa at temperatures ranging from about 125xc2x0 to 175xc2x0 to form self-lubricating ceramic composites capable of maintaining hot-hardness temperatures above 750xc2x0 C.
It is therefore an object of this invention to provide a process of preparing self-lubricating ceramic composites of low porosity capable of maintaining a hot-hardness at temperatures greater than 750xc2x0 C.
It is another object of this invention to provide a process of preparing self-lubricating ceramic composites of low porosity from a slurry of castable, cementitious ceramic powders.
It is further an object of this invention to provide self-lubricating ceramic composites of low porosity from castable cementitious ceramics capable of being formed into rigid structures by using known casting techniques.
The foregoing and additional objects associated with this invention will become more apparent from the detailed description, and with reference to the drawings; wherein like elements are designated by like reference numerals.